Method of nitrating organic compounds



Patented Feb. 3, 1948 METHOD OF Nr'rrcA'rmG ORGANIC COMPOUNDS? AVain-an- Kokatniir, ,New York, N. Y., assig nor to Autoxygen, Inc NewYork, N. Y., a corporaftionofiN'ew' York No Drawing.-

Application December 5, 1940; SerialNo. 368,614 1 ioiaimsp (01.260-467)- This invention relates to. a method oftn trating organiccompounds and particularly aromatic organic compounds; H g a I V H p tThe nitration of organiccompounds is one of the most important; unit;processes and playszan important A Part in :the dyestufi industry andthe manufacture of explosives. Water is a product of the reaction ofthe: nitration process, and unless this Water is removed, the; reactionapproaches an equilibrium before completion. Many attempts have been'made to remove this water by chemical combination and thus to preventan equilibrium being produced, so that the reaction would be continued.-The' most successful of such attempts and the one now commonly in useis to provide: concentrated sulphurieacid in the zone of the :reactiontoabsorb the water produced. The sul'phuric: acid ismixed' with nitricacid in certain definite proportions; depending upon the particularmaterials used; and this mixture of nitric andsulphuric acids isreferred to in the art as "mixed nitrating'acidr r a For. mononitrationthe proportion of nitric in the mixed acid generally does not exceed33%. For higher nitration the proportionof nitric acid becomes smallerand smallerand may be as low as 3 to5-%= with consequentdncreases in theamount of sulphuric acid. The increased proportion of sulphuricacidisnecessary to combine with the largeramount of water-evolved duringthese particular reactions; Thusthe mixed acid requires a certaindistribution of the two acids depending upon the particularwnitrationreaction involved before itcan". be used for that reaction:

It iswell known in the art that the speedor nitration of an organiccompound is direetly pro portionate to the concentration of nitricanhydride, or N205, which is nitric acid without'any Water. It willthusbe seen that nitration reactime using mixedacidsare at best ineflicient;since the proportion of nitric acid inthe mixture of acids is a minorfraction:

Furthermore, it has not been possibleheretofore toremove the-water ofvreaction fromthe zone of reaction.- This Iihavefound to be a seriv 2 A.7 mg- The effect of superheatingthe mass is to produce highernitrations as well as to cause oxi dation which gives rise to serioushazards, ofex plosion because of. the sensitivity of the reactionmixture, A r Y It'is, therefore, a primary object of thepresentinvention to provide a process of nitration which will not have any 'ofthe disadvantages mentioned above and by means of which water may beremoved from the zone of reaction without the necessity for usingsulphuric acid.

Another important object of the invention is to provide a process ofnitration by means or which nitro compounds may be produced more.economicalh; than has been possibleheretofore. v Another objector theinvention isto provide a process for producingpurer nitro-compounds,particularly purerpolynitro compounds with better yield than hasheretofore been known,

Another object 'oi the invention is to provide a safer nitration processthan has heretofore been used. H Still another object of the inventionis toprovide a process of nitration by which the reaction temperaturemay be controlled so as to reduce the effect of local superheating.

A further object of the invention is to provide anitration process whichwill reduce superheatingofthe reaction mass, causedby exothermicevolution of heat, by distributing this heat molecularly throughout theentire reaction mass. v U Other objects of the invention will in part beobvious and will inpart appear here nafter, A

p The invention accordingly comprises the sev-r eral steps and therelation of one or more of such steps withrespect to each of the othersthereof, which will be exemplifiedin the process hereinafter disclosed,and the scope of the application of which will be indicated in theclaims 'I have discovered that the above mentioned Ob! jects may beattained and the disadvantages found in processes of the prior art.reduced or eliminated by carrying on the reaction of nitration; incombination with a diluent which is substantially inert to the, reactingmaterials, is. substantially immiscible with water, has a suitableboiling point, andis preferably a solvent of the compound to be nitratedas well as the product of nitration. g p v V t .In carryingouttheprocess the diluent is mixed with the particular material tobeqnitrated and the nitrating agent and the temperature adjusted. untilth mixture begins to boil. A diluent with a boiling. point approximatingthe desired reaction temperature is selected, so that by main--- tainingthe mixture at the boiling point the reaction will be carried n and atthe same time water evolved by the reaction or present in the mixturewill distill out with the diluent.

This Water may be carried off with the diluent in accordance with theprinciple of partial pressure distillation. By this I mean that thevapor pressures of Water and the diluent are sufficient together toovercome the external pressure, thus permitting the water to vaporize ata temperature under that normally required as a result of the azeotropicmixture formed. As distillation takes place, the water is removed as itis evolved, together with the diluent, and the process is carried onuntil the reaction is completed.

In some cases the diluent may be the material which is to be nitrated,which then may be supplied in excess, and in other cases a separatematerial is used for the diluent. Suitable diluents, which may be usedfor my purpose, are saturated hydrocarbons of the aliphatic series;cycloparaffins; nitroparaiiins; 7 other organic compounds that do notreact with the nitrating agent, including ethers, such as dibutyletherand isopropylether, esters, such as methyl acetate, ethyl formate,methyl nitrate, ethyl nitrate, and propyl nitrate, and aliphaticchlorine compounds, such as chloroform, dichlorethylether and carbontetrachloride; aromatic hydrocarbons (when used in excess only) andaromatic nitro-compounds (for nitrations of the same order only). 7

Certain practical considerations are important to mention in connectionwith carrying out my invention. Generally speaking, nitration operationsare carried on at temperatures ranging from room temperatures to thoseas high as 130 C. Polynitro-compounds explode at temperatures higherthan 175 C. For this reason I prefer to use diluents, the end boilingpoints of which do not exceed about 175 C. As the reaction is to becarried on at the boiling point of the diluent in order to remove theWater from the zone of reaction, the ratio of the amount of diluent tothat of water removed may be inordinately high in cases where thediluent boils at or below the temperature of reaction. To avoid this, Imay use a diluent that normally boils higher than the reactiontemperature and make it boil at the temperature of reaction byreducingthe pressure. In such a case, I may use diluents that normallyboil as high as 200 C. or even higher. It should be definitelyunderstood, however, that no diluent which boils above 150 C. at apressure of 29 inches of mercury (atmospheric pressure), should be usedat that pressure.

By carrying on the reaction at a temperature at which the diluent andwater will vaporize together, the temperature of reaction can beautomatically regulated and the water evolved by the reactionprogressively removed, as it is formed, from the zone of reaction bydistillation.

The diluent further acts as a safety valve and as a medium of molecularcooling in the distribution of exothermic heat evolved during allnitration reactions. Since the reactant as well as the product ispreferably soluble in the diluent, any heat evolved during the reactionis distributed throughout the mass of the solution. If thereaction massbecomes too hot, the diluent will boil ofi vigorously and thus give anindication that the reaction is going too fast.

The nitro-compounds in general have acidic properties and the larger thenumber of nitrogroups in the compound, the greater will be the acidity.Because of the aciditl, Of 2 1 @993 4 pounds, salts are formed withmetals for bases. These salts are very sensitive and are looked upon asthe primary cause of the explosions of many nitro-compounds. In theproduction of explosives such as TNT (trinitrotoluol) or picric acid(trinitrophenol), special metal is used in the apparatus for carryingout nitration reactions so that the formation of salts may be avoided.With ionizing solvent. This is another reason why my process has greatersafety than those heretofore used.

Of course, by not using sulphuric acid in my process, I completelyeliminate the necessity for recovering the spent acid and the attendantmany washings and neutralizations that must be carried on to use thisacid over again. Economies in the use of raw materials and handling arethus afiected.

In order to illustrate my invention the followin examples are given:

Example I .'Nitrobenzene Approximately 1,000 parts of benzene are mixedwith not more than 500 parts of nitric acid, or an equivalent quantityof nitric acid of any specific concentration. The mixture is placed in avessel fitted with a distilling column or reflux condenser, a watertrap, and an agitator. The benzene isused not only as the compound to benitrated but also as the diluent; hence an excess of benzene is used andthis excess also acts as a control for preventing nitration further thanthe mononitrate stage.

Benzene and water when mixed together boil at about 70 C. and hence thisbecomes the reaction temperature. As the nitrobenzene is formed andaccumulates in'the reaction vessel, the boiling point of the liquidgradually increases but the temperatures are not allowed to go higherthan 100 C. If the'temperature starts to rise above 100 C., the pressureabove the system is reduced sufficiently to maintain the temperature ofthe liquid at or below 100 C. As the reaction mixture is maintained at atemperature such that the benzene and water keep on boiling under thereflux condenser, water vapor will pass into the condenser with thebenzene vapor and be separated in the trap and can be measured. When theamount of Water found in the trap corresponds to the amount of watertheoretically calculated to be evolved in the reaction, the reaction isconsidered completed. The product in the reaction chamber is 'a mixtureof nitrobenzene dissolved in the excess of benzene.

If the benzene is not absolutely pure; or if there'are any impurities inthe reaction mixture, a small fraction of brown gas which is nitricperoxide, N204, is evolved. This can berecovered by absorption in waterand hence is no loss.

The reaction takes place quantitatively and nitrobenzene with a yield ofabout 99% is produced. The reaction product can be distilled to removethe excess benzene and then steam-distilled to produce light yellowcolor mononitrobenzene, or it may be purified and isolated in anyconventional manner.

.of 50% nitric acid or the equivalent of any other percent concentrationis added. The mixture is placed in a distilling vessel fitted with areflux condenser column, an agitator, and a water trap. The distillingvessel is then heated in any desired manner until the petroleum etherbegins to boil and pass ofi as vapor, carrying the water with it. Theheating is continued until all the free water as well as' the water ofnitration is trapped in.

the water trap. The reaction is considered complete when the'amount ofwater trapped is approximately the same as the amount theoreticallycalculated to be evolved in the reaction plus that originally present. 4

When the mass is allowed to cool, the product, alpha nitronaphthalene,crystallizes out from the petroleum ether solution. If the nitric acidused is suflicient to convert all the naphthalene into nitronaphthalene,no free naphthalene will be found in the reaction product'which willconsist primarily of alpha nitronaphthalene and petroleum ether. Thenitronaphthalene maybe purified and recovered in any conventionalmanner.

Example III (m-dinitrobenzene) Approximately 1,230 parts ofmononitrobenzene, as produced in Example I, or produced in any otherknown manner, are mixed with about 1000 parts of a saturated hydrocarbonoi the petroleum series, boiling between 100 and 120 C., which acts asthe diluent. To this mixture is added approximately 630 parts of 100%nitric acid or an equivalent amount of nitric acid of any otherconcentration. The mixture is placed in a distilling vessel fitted witha reflux condenser column, an agitator, and a water trap. The mixture isheated in any conventional manner and agitated until the diluent beginsto boil off carrying the water with it. The temperature is heldsubstantially constant to boil the hydrocarbon and Water together. Thereaction is continued until all the water of reaction as well as anywater of dilution, if any, is collected in the trap. The reaction isthen considered complete, since substantially all the nitric acid isused up in the nitration, and there remains one layer ofm-dinitrobenzene in the diluent. The m-dinitrobenzene may becrystallized out from solution or may be separated by boiling off thediluent, or in any other conventional manner. The melting point or them-dinitrobenzene is between 83 and 85 C. The product thus produced ispurer than the product produced by other known processes.

Example IV (nitroglycerine) About 210 parts of nitric acid of 100%concentration, or an equivalent amount of any concentration above 80%,are mixed with about 500 parts of chloroform in a still fitted with anagitator, a feeding device, and a reflux water trap.

The apparatus is placed under sumcient vacuum the free waterand thatliberated by the reactioncollected in the water trap. The solution maythen be washed with Water or alkaline solution to remove any dissolvedacid and the product stored in the solution or isolated by evaporatingthe chloroform. The yield is over 95% of that calculated theoretically.

Example V (trinitrotoluene) .About 460 parts of toluene, about 1000parts of nitric acid of 95% concentration, or the equivalent amount ofany other concentration, and? about 1500' parts of benzine or lightgasolene are placed in a still fitted with an agitator, and. a refluxwatertrap. The still is heated and the: mixture agitated until thediluent, the benzine, carrying water with it, begins to reflux. Thereaction is continued. until all of the free water and that liberated bythe reaction is collected in the water trap. When this happens thenitric acid layer has all disappeared and is completely absorbed. Thisis considered the end of the reaction. On coolingtrinitrotoluenecrystallizes out. However, it is preferableto wash the solution with hotwater or hot alkaline solution and then evaporate the dry solution tocrystallize the bulk of the trinitrotoluene. The mother liquorcontaining the residual trinitrotoluene may be used for the nextoperation as a diluent. By this procedure very pure trinitrotoluene isobtained. If this is not desired, all the diluent may be removed byevaporation, leaving trinitrotoluene residue which may be treated in anyconventional manner. If high boiling diluents are used, the reactionshould be conducted in sufficient vacuum so that the diluent boils at atemperature not to exceed 100 C. Preferred reaction temperature isbetween and C. Yield is of the order of 90% of that calculatedtheoretically.

In all examples the diluent returns to the reaction vessel owing to itscondensation in the reflux condenser. As it vaporizes, however, itconstantly carries the water away from the zone of reaction and allowsit to collect in the trap. It is also to be clearly understood that allthe reactions in all of the examples are carried on with agitation.

Since certain changes may be made in carrying out the above methodwithout departing from the scope of the invention, it is intended thatall matter contained in the above description will be interpreted asillustrative and not in a limiting sense.

It is also to be understood that the following claims are intended tocover all the generic and specific features of the invention hereindescribed, and all statements of the scope of the invention which as amatter of language might be said to fall therebetween.

Having described my invention, what I claim as new and desire to secureby Letters Patent is:

1. The process of nitrating an organic volatile nitratable compoundcapable of forming an azeotrope with Water, consisting of the steps ofmixing an excess of said compound with nitric acid,

